We are studying the genetic organization of polyoma virus and the role of viral gene functions in cell growth regulation. We have shown that two classes of polyoma mutants defective in transformation (tsA mutants and host range non-transforming mutants) are able to complement for transformation. We will extend the experiments by testing which viral genomes are present in cells transformed after mixed infection by the two classes of mutants. We have shown that a late temperature-sensitive mutant of polyoma is altered in the 30K dalton virion protein. We will study the possible role of this virion protein in regulating viral messenger RNA transcription.